Time average holography is a technique which applies holographic interferometry to the study of the surface deformation of vibrating objects. The technique involves photographically recording the interference pattern between a coherent light source and light reflected from a vibrating object illuminated by the coherent light. U.S. Pat. No. 3,645,129 and commonly assigned U.S. Pat. No. 4,408,881 disclose methods for testing the integrity of bonded joints between two members by time average holography. The disclosure of commonly assigned as U.S. Pat. No. 4,408,881 is disclosures incorporated herein by reference. In each of these methods a hologram of the bonded joint is generated while the joint is being vibrated. If there is a discontinuity in the bond, the two members vibrate independently of each other and a number of superimposed interference fringes appear against the background of the hologram as a contrasting area. The method is useful as a non-destructive quality control test for articles having bonded joints.
In practice the method has been found to be unreliable in some applications because of an inability to distinguish between indications associated with disbonds and those associated with other causes. Consequently, a significant number of false positive readings may result from the conventional method.